The invention concerns an improved method and stabilized reagent for the photometric determination of creatine kinase in biological samples such as in particular human blood serum or plasma. The reagent is essentially characterized in that it contains a substoichiometric amount of an organic or inorganic reducing sulphur compound.
The determination of creatine kinase in serum or plasma plays an important role in the diagnosis of cardiac infarction. A photometric test is used as a standard method for this in which adenosine 5xe2x80x2-triphosphate (ATP) and creatine are generated in coupled enzymatic reactions from creatine phosphate and adenosine 5xe2x80x2-diphosphate (ADP) by the creatine kinase (CK) contained in the sample; the ATP is for example used to form glucose-6-phosphate from glucose in the presence of hexokinase (HK) which is oxidized to gluconate-6-phosphate in a reaction catalysed by glucose-6-phosphate dehydrogenase (G6P-DH) while simultaneously converting NAD+ or NADP+ into NADH or NADPH: 
The measured quantity is the increase in absorbance caused by the formation of NAD(P)H in a specified time interval at a certain temperature, usually between 25 and 37xc2x0 C., which is proportional to the CK activity in the sample volume. In order for the CK to develop its full enzymatic activity, the determination is usually carried out in the presence of CK activators for example thiol compounds such as glutathione, dithiothreitol, thioglycerol, 2-mercaptoethanol and N-acetyl cysteine.
Furthermore inhibitors for myokinases (adenylate kinase) that may be present in the sample such as adenosine-5xe2x80x2-monophosphate (AMP) and/or di-adenosine-pentaphosphate are preferably also added to the reagent for the CK test. Despite these additives unspecific ATP can be formed from ADP according to the following reaction: 
This can lead to an additional formation of NAD(P)H which falsifies the CK determination. Interference by incompletely inhibited adenylate kinase which can occur especially in haemolytic samples can be eliminated by determining the activity of adenylate kinase before actually starting the CK reaction by adding creatine phosphate and subtracting this (so-called rate blanking) from the total activity (CK and adenylate kinase). In this connection it is important that all components required to detect the ATP quantity formed by adenylate kinase are present in the first reaction solution (except for creatine phosphate).
Furthermore specific inhibitors such as antibodies directed against particular CK isoenzymes can also be added to the reagent for the determination of creatine kinase isoenzymes.
In order to increase the detection sensitivity the reagent forming glucose-6-phosphate and converting glucose-6-phosphate by means of NAD(P)+ and G6P-DH can optionally additionally contain 6-phosphoglucono-lactonase and gluconate-6-phosphate dehydrogenase; in this case two moles of NADH or NADPH are generated per mole of ATP or glucose-6-phosphate formed (R. Vormbrock and R. Helger, Enzyme 38, Suppl. 1 (1987), p. 20/21).
In addition it is for example possible to carry out the CK test by converting the ATP formed from creatine phosphate and ADP into glycerol-3-phosphate using glycerol and glycerol kinase, preferably in the presence of magnesium ions, which is converted enzymatically in the presence of oxygen into hydrogen peroxide and is detected in the usual manner by means of peroxidase and redox indicators.
In principle all components required for the CK determination, i.e. enzymes and substrates, can be present in a single reagent. However, especially on analyzers the determination is preferably carried out by firstly preincubating the sample for several minutes with a first partial reagent which, apart from creatine phosphate, contains all components necessary for the detection reaction and other potential auxiliary substances such as N-acetyl cysteine, adenylate kinase inhibitors and optionally CK isoenzyme inhibitors and subsequently the detection reaction is started by adding a second partial reagent which essentially contains creatine phosphate in a buffered solution.
In addition to avoiding formation of NAD(P)H by CK during the phase in which the enzyme is activated in the presence of the added thiol compound, this also prevents possible falsification of the measured result by excessive activities of adenylate kinase that cannot be adequately inhibited by adenylate kinase inhibitors which can for example occur in haemolytic samples. For this purpose a first measurement of the rate of formation of NAD(P)H is carried out after mixing the sample and the first partial reagent and the result is subtracted from the rate of formation of NAD(P)H after adding the second partial reagent containing creatine phosphate.
A disadvantage for the user of such reagents is in particular that the reagents have to be firstly prepared before use by dissolving the solid components e.g. lyophilisates, granulates or tablets and moreover they are only stable for a few days or weeks even when stored cold between 2 to 8xc2x0 C. As a result of rationalization in the clinical laboratory there is nowadays an increasing need for ready-to-use reagents with a shelf-life of at least 12 months at 2 to 8xc2x0 C. At present this demand cannot be met due to lack of stability. The instability of the ready-to-use reagent is mainly caused by the instability of N-acetyl cysteine (NAC, activator of CK).
Although the stability problem can be resolved as described in EP 0 686 561 by storing NAC (together with NAD(P)) in a second solution at pH 3.0, this has considerable disadvantages. In this reagent formulation the CK is only activated when the CK reaction is started. Hence a longer waiting period is required before the actual start of the measurement in order to avoid the lag phase and this considerably limits the measuring range of the method. A further disadvantage is that it is not possible to eliminate the adenylate kinase interference by rate blanking. Measures for stabilizing liquid reagents suitable for the determination of CK are described in EP 0 774 514 such as the addition of a phosphine and a sulfhydryl compound such as dithiothreitol, DTT. However, a disadvantage of adding DTT is that it destabilizes the auxiliary enzyme G6P-DH and thus necessitates the addition of an additional component in the form of a hydroxylamine compound (for example carboxymethoxylamine hydrochloride) to stabilize the G6P-DH (EP 0 640 686).
Furthermore EP 0 721 986 describes stable reagents for the determination of creatine kinase which contains certain SH compounds such as thiolglycerol (TG), 2-mercaptoethanol (2ME) or 2-mercaptoethanesulfonic acid (2MES) in molar concentrations. In contrast to other activators these do not inhibit the CK activity in their oxidized form. However, relatively high concentrations of such SH compounds can have undesired side-effects such as the inactivation of auxiliary enzymes (e.g. G6P-DH). Consequently these activators are also at present not recommended by experts for the determination of the CK activity (e.g. according to the IfcC, DGkCh). Furthermore the comparability of such test systems is open especially with regard to the isoenzymes and CK from different species.
Consequently and especially to further rationalise work in the clinical laboratory under increasing cost pressure, there is today an increasing requirement for stabilized reagents which can be stored and are stable in a ready-to-use liquid form for at least 12 months at 2 to 8xc2x0 C. without a renewed calibration and which largely eliminate the risk of unspecific reactions caused by certain additives.
This object is achieved by a reagent which contains a suitable buffer system, substrates and corresponding coenzymes that can be converted by CK, a CK activator and components required for one or several subsequent enzymatic reactions and an organic or inorganic sulphur compound or mixtures of corresponding sulphur compounds in a submolar amount relative to the CK activator. Appropriate CK activators are known to a person skilled in the art. A reagent is preferred according to the invention which contains creatine phosphate and adenosine 5xe2x80x2-diphosphate (ADP) as the substrates that can be converted by CK. A reagent is particularly preferred which contains glucose, an enzyme that forms glucose-6-phosphate such as hexokinase, ADP, creatine phosphate, a CK activator, optionally adenosine 5xe2x80x2-mono-phosphate, a coenzyme in an oxidized form such as NAD+, NADP+ or derivatives thereof, a G6P dehydrogenase and optionally one or several adenylate kinase inhibitors and/or other activators and a substoichiometric amount of one or several of the said sulphur components which are preferably reducing substances with a negative charge. In particular sulphur-oxygen compounds such as thiosulfate, dithionite, pyrosulphuric acid, polythionic acid and salts thereof are added according to the invention. Inventive salts that come into consideration are in particular oxygen acids of sulphur such as thiosulfates or sulfites of alkali and/or alkaline earth ions or sulfite-releasing compounds. Corresponding disodium salts have proven to be particularly advantageous for stabilizing reagents for the determination of CK.
The concentration of the inventive sulphur component in the reagent is usually of the same order of magnitude as that of the enzyme (CK) to be determined and is no more than 20 mmol/l. Concentrations of the sulphur component between ca. 0.01 and 5 mmol/l are preferred and 0.1 to 1.0 mmol/l is especially preferred.
All other components and optionally other auxiliary substances including the optionally added 6-phospo-gluconolactonase and gluconate-6-phosphate dehydrogenase are used in the usual concentrations for a person skilled in the art.
A particular embodiment of the reagent according to the invention is when the components required for the CK determination are divided between two separate partial reagents whereby the first partial reagent contains all substances and enzymes required for the CK determination apart from creatine phosphate and the second partial reagent contains creatine phosphate and for example glucose. The sulphur component according to the invention and the CK activator can be present in the first and/or second reagent. It has proven to be advantageous to not add the sulphur component, e.g. via the second reagent solution, to the more complex first reagent which also contains the CK activator until immediately before the measurement.
A further special embodiment of the invention is a reagent which contains glucose in combination with NAD(P)+ and G6P-DH in a partial reagent at a low concentration preferably of ca. 5 mmol/l or less; ca. 0.5 to 1.0 mmol/l glucose is preferred.
The aqueous reagent according to the invention is characterized by exceptional stability after long storage between 2 and 8xc2x0 C. i.e. over periods ranging from several months to 12 months without renewed calibration and by a high specificity. Thus in a test of CK function only a slight deviation was found compared to a freshly prepared reagent, i.e. the CK recovery rates were approximately 100%.
A further subject matter of the invention is a method for the enzymatic determination of creatine kinase in a biological sample material in the presence of one of the organic or inorganic sulphur compounds that are specified above in more detail.